The present invention relates to a method for making a heterostructure notably intended for applications in the fields of electronics, optics, photovoltaics, or optoelectronics.
This is more specifically a method that comprises the following steps:                implantation of atomic species inside a first so-called “donor” substrate, so as to form an embrittlement area therein,        assembly of a second so-called “receiver” substrate on the donor substrate,        detachment of the rear portion of said donor substrate along the embrittlement area, so as to individualize a thin layer of interest on the receiver substrate.        
A method of this kind is generally known under the trademark of SMARTCUT®.
By means of this technique, the specific advantages of the materials of the thin layer and of the support may be combined.
By transferring the layer, it is possible to associate in a same stack, portions which a priori have incompatibilities such as a large difference in the thermal expansion coefficient.
With such structures, one of the problems encountered during their manufacturing is due to the presence of a very strong stress field in the various layers in contact during a heat treatment such as the one during which detachment of the thin layer occurs. These stresses are due to the differential thermal expansion between the materials put into contact.
In such a case, it is essential to perform detachment at a lower temperature than the critical temperature at which the heterostructure will be degraded because of the stresses mentioned above. As the value of the stresses coming into play is related to the thickness of the assembled substrates, it will be noted that after detachment of the layer, the structure may then be subject to more significant heat treatments.
The degradation that may occur during the manufacturing of the final structure is typically breakage of both substrates put into contact with each other and/or their detachment at the bonding interface.
Thus, in the case of the heterostructure of the silicon-on-sapphire (Al2O3) type, the thermal expansion coefficients (TECs) of silicon and of sapphire are 3.6×10−6/° C. and 5.0×10−6/° C., respectively. The differential expansion phenomenon first of all occurs at the moment of the heat treatment step intended to cause detachment of the thin layer from the silicon donor substrate.
In such a TEC difference situation, the transfer of the thin layer becomes delicate because the heat treatment steps (such as, for example, detachment annealing), which takes place after adhesive bonding and before transfer, are necessarily limited. Indeed, the different expansion of both assembled substrates may lead to detachment of the assembly or to breakage of the substrate.
The invention is aimed at improving this state of the art.
More particularly, the invention is based on the observation, within the scope of a layer transfer method, notably by the SMARTCUT® technology, that during the assembling of a donor silicon substrate with a sapphire receiver substrate (and more generally when the thermal expansion coefficient (TEC) of a donor substrate is less than the TEC of a receiver substrate), the donor substrate is tensioned during heat treatments, which follow the assembling and lead to fracture.
Now, it seems that the tensioned condition of the material of the donor substrate, in particular at the implanted area, is an obstacle to the propagation and/or development of “platelets” and other microscopic defects, precursors of the fracture phenomenon. More specifically, the orientation of the defects in this stressed material does not seem to occur along a beneficial direction, i.e., in the plane of the plate. This obstacle leads to faulty, delayed fracture or even completely blocked fracture.